Methods and systems for operating a vehicle with two fuel tanks

ABSTRACT

Systems and methods for operating a fuel system that includes two separate fuel tanks are disclosed. In one example, fuel may be purged from a fuel rail in response to Reid vapor pressure of a fuel so that engine starting may be improved. A fuel with a higher Reid vapor pressure may be pumped into the fuel rail when the engine is expected to be cold started.

FIELD

The present description relates to a system and methods for operating avehicle that includes two fuel tanks. The system and methods may beuseful to improve engine starting.

BACKGROUND AND SUMMARY

A vehicle may be equipped with two fuel tanks to extend the vehicle'soperating range and to provide an additional source of energy foroperating auxiliary loads that may be coupled to the vehicle. Thevehicle's operator may selectively choose which tank fuel may be drawnfrom to supply the vehicle's engine. The vehicle's operator may leavefuel in one of the fuel tanks a longer amount of time than the amount oftime that fuel is left in the vehicle's other fuel tank. Consequently,fuel in one of the vehicle's fuel tanks may be older than fuel in thevehicle's other fuel tank. Short chain hydrocarbons may tend toevaporate from the older fuel causing the Reid vapor pressure of theolder fuel to be reduced. In addition, greater amounts of water may havecondensed in the older fuel. As a result, the engine may not start aswell as may be expected if the engine is started with the older fuel.Consequently, engine emissions may increase.

The inventors herein have recognized the above-mentioned disadvantagesand have developed a method for operating a fuel system, comprising: inresponse to an indication of an impending engine start and an estimatedReid vapor pressure of a first fuel exceeding an estimated Reid vaporpressure of a second fuel, flushing the second fuel from a fuel rail toa second fuel tank with the first fuel supplied from a first fuel tankvia a controller.

By flushing a fuel having a lower Reid vapor pressure from a fuel railwith a fuel having a high Reid vapor pressure before a cold enginestart, it may be possible to improve engine starting and emissions.Specifically, the engine may be started with the fuel with the higherReid vapor pressure when the engine is cold so that fuel injected intothe engine may have a better chance of vaporizing and combusting ascompared to if the engine were started with a fuel with a lower Reidvapor pressure. Conversely, if the engine is hot started, a fuel with ahigher Reid vapor pressure may be flushed from the fuel rail andreplaced by a fuel with a lower Reid vapor pressure so that engineemissions may be maintained at lower levels.

The present description may provide several advantages. In particular,the approach may reduce engine emissions. Further, the approach mayimprove engine smoothness during cold engine starts. In addition, theapproach may help to remove older fuel from a fuel system so thatfresher fuel may be available to operate the engine with.

The above advantages and other advantages, and features of the presentdescription will be readily apparent from the following DetailedDescription when taken alone or in connection with the accompanyingdrawings.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages described herein will be more fully understood by readingan example of an embodiment, referred to herein as the DetailedDescription, when taken alone or with reference to the drawings, where:

FIG. 1 shows a schematic diagram of an engine in a vehicle;

FIG. 2 shows a schematic diagram of a fuel system;

FIG. 3 shows an example engine operating sequence according to themethod of FIGS. 4 and 5; and

FIGS. 4 and 5 show a method for operating a fuel system of an internalcombustion engine.

DETAILED DESCRIPTION

The present description is related to operating a fuel system for aninternal combustion engines. The fuel system includes two fuel tanks forextending the operating range of a vehicle and a run time of an engine.The engine may be of the type shown in FIG. 1, or alternatively, adiesel engine. The fuel system may be configured to exchange fuelbetween two fuel tanks. The fuel system may be of the type shown in FIG.2. The fuel system may be operated as shown in the sequence of FIG. 3according to the method of FIG. 4.

Referring to FIG. 1, internal combustion engine 10, comprising aplurality of cylinders, one cylinder of which is shown in FIG. 1, iscontrolled by electronic engine controller 12. Engine 10 is included invehicle 80, which has one or more doors 82 for ingress and egress.Vehicle 80 may also include a fob, phone, or other remote device 84 thattransmits a signal directly or indirectly to vehicle 80 and controller12 to indicate when a passenger is proximate to vehicle 80.

Engine 10 includes combustion chamber 30 and cylinder walls 32 withpiston 36 positioned therein and connected to crankshaft 40. Flywheel 97and ring gear 99 are coupled to crankshaft 40. Starter 96 includespinion shaft 98 and pinion gear 95. Pinion shaft 98 may selectivelyadvance pinion gear 95 to engage ring gear 99. Starter 96 may bedirectly mounted to the front of the engine or the rear of the engine.In some examples, starter 96 may selectively supply torque to crankshaft40 via a belt or chain. In one example, starter 96 is in a base statewhen not engaged to the engine crankshaft. Combustion chamber 30 isshown communicating with intake manifold 44 and exhaust manifold 48 viarespective intake valve 52 and exhaust valve 54. Each intake and exhaustvalve may be operated by an intake cam 51 and an exhaust cam 53. Theposition of intake cam 51 may be determined by intake cam sensor 55. Theposition of exhaust cam 53 may be determined by exhaust cam sensor 57.

Direct fuel injector 66 is shown positioned to inject fuel directly intocylinder 30, which is known to those skilled in the art as directinjection. Port fuel injector 67, injects fuel to intake port 69, whichis known to those skilled in the art as port injection. Fuel injector 66delivers liquid fuel in proportion to a voltage pulse width or fuelinjector pulse width of a signal from controller 12. Likewise, fuelinjector 67 delivers liquid fuel in proportion to a voltage pulse widthor fuel injector pulse width from controller 12. Fuel is delivered tofuel injectors 66 and 67 by a fuel system (not shown) including a fueltank, fuel pump, and fuel rail (not shown). Fuel is supplied to directfuel injector 66 at a higher pressure than fuel is supplied to port fuelinjector 67. In addition, intake manifold 44 is shown communicating withoptional electronic throttle 62 which adjusts a position of throttleplate 64 to control air flow from air intake 42 to intake manifold 44.In some examples, throttle 62 and throttle plate 64 may be positionedbetween intake valve 52 and intake manifold 44 such that throttle 62 isa port throttle.

Distributorless ignition system 88 provides an ignition spark tocombustion chamber 30 via spark plug 92 in response to controller 12.Universal Exhaust Gas Oxygen (UEGO) sensor 126 is shown coupled toexhaust manifold 48 upstream of catalytic converter 70. Alternatively, atwo-state exhaust gas oxygen sensor may be substituted for UEGO sensor126.

Converter 70 can include multiple catalyst bricks, in one example. Inanother example, multiple emission control devices, each with multiplebricks, can be used. Converter 70 can be a three-way type catalyst inone example.

Controller 12 is shown in FIG. 1 as a conventional microcomputerincluding: microprocessor unit 102, input/output ports 104, read-onlymemory 106 (e.g., non-transitory memory), random access memory 108, keepalive memory 110, and a conventional data bus. Controller 12 is shownreceiving various signals from sensors coupled to engine 10, in additionto those signals previously discussed, including: engine coolanttemperature (ECT) from temperature sensor 112 coupled to cooling sleeve114; a position sensor 134 coupled to a propulsive effort pedal 130 forsensing force applied by a human foot 132; a position sensor 154 coupledto brake pedal 150 for sensing force applied by a human foot 152, ameasurement of engine manifold pressure (MAP) from pressure sensor 122coupled to intake manifold 44; an engine position sensor from a Halleffect sensor 118 sensing crankshaft 40 position; a measurement of airmass entering the engine from sensor 120; and a measurement of throttleposition from sensor 58. Barometric pressure may also be sensed (sensornot shown) for processing by controller 12. In a preferred aspect of thepresent description, engine position sensor 118 produces a predeterminednumber of equally spaced pulses every revolution of the crankshaft fromwhich engine speed (RPM) can be determined.

In some examples, the engine may be coupled to an electric motor/batterysystem in a hybrid vehicle. Further, in some examples, other engineconfigurations may be employed, for example a diesel engine withmultiple fuel injectors. Further, controller 12 may receive input andcommunicate conditions such as degradation of components to light, oralternatively, human/machine interface 171.

During operation, each cylinder within engine 10 typically undergoes afour stroke cycle: the cycle includes the intake stroke, compressionstroke, expansion stroke, and exhaust stroke. During the intake stroke,generally, the exhaust valve 54 closes and intake valve 52 opens. Air isintroduced into combustion chamber 30 via intake manifold 44, and piston36 moves to the bottom of the cylinder so as to increase the volumewithin combustion chamber 30. The position at which piston 36 is nearthe bottom of the cylinder and at the end of its stroke (e.g. whencombustion chamber 30 is at its largest volume) is typically referred toby those of skill in the art as bottom dead center (BDC). During thecompression stroke, intake valve 52 and exhaust valve 54 are closed.Piston 36 moves toward the cylinder head so as to compress the airwithin combustion chamber 30. The point at which piston 36 is at the endof its stroke and closest to the cylinder head (e.g. when combustionchamber 30 is at its smallest volume) is typically referred to by thoseof skill in the art as top dead center (TDC). In a process hereinafterreferred to as injection, fuel is introduced into the combustionchamber. In a process hereinafter referred to as ignition, the injectedfuel is ignited by known ignition means such as spark plug 92, resultingin combustion. During the expansion stroke, the expanding gases pushpiston 36 back to BDC. Crankshaft 40 converts piston movement into arotational torque of the rotary shaft. Finally, during the exhauststroke, the exhaust valve 54 opens to release the combusted air-fuelmixture to exhaust manifold 48 and the piston returns to TDC. Note thatthe above is shown merely as an example, and that intake and exhaustvalve opening and/or closing timings may vary, such as to providepositive or negative valve overlap, late intake valve closing, orvarious other examples.

Referring now to FIG. 2, a schematic view of a fuel system 200 forinternal combustion engine 10 is shown. Fuel system 200 includes a firstfuel tank 202 that includes a first fuel pump. First fuel tank iscoupled to three-way fuel supply valve 206 via a conduit or pipe 203.Three-way fuel supply valve 206 is also coupled to fuel rail 208 viaconduit or pipe 205. Fuel rail 208 may supply fuel to fuel injectors 66or 67 as shown in FIG. 1. Fuel rail 208 is coupled to a fuel returnvalve 210 via conduit or pipe 209. Fuel return valve 210 is coupled tofirst fuel tank 202 via conduit or pipe 211. Fuel return valve 210 isalso coupled to second fuel tank 212 via conduit or pipe 213. Secondfuel tank 212 includes a second fuel pump 214. Second fuel pump 212 iscoupled to three-way fuel supply valve 206 via conduit or pipe 217.

Three-way fuel supply valve 206 is shown in a default position wherefuel is allowed to flow from first fuel pump 204 to fuel rail 208.Three-way fuel supply valve 206 may allow fuel to flow from second fuelpump 214 to fuel rail 208 when three-way fuel supply valve 206 is in asecond position.

Fuel return valve 210 is shown in a default position where fuel flowbetween fuel rail 208 and first fuel tank 202 is not permitted. Fuelflow between fuel rail 208 and second fuel tank 212 is not permittedwhen fuel return valve 210 is in the default position. Fuel may flowfrom fuel rail 208 to first fuel tank 202 when fuel return valve 210 isin a second position. Fuel may flow from fuel rail 208 to second fueltank 212 when fuel return valve 210 is in a third position.

The system of FIGS. 1 and 2 provides for a system, comprising: a firstfuel tank; a second fuel tank; a first valve coupled to the first fueltank and the second fuel tank; a second valve coupled to the first fueltank and the second fuel tank; an internal combustion engine including afuel rail; and a controller including executable instructions stored innon-transitory memory that cause the controller to route a first fuelfrom the first fuel tank through the first valve to the fuel rail androute a second fuel from the fuel rail through the second valve to thesecond fuel tank in response to an indication of an impending start ofthe internal combustion engine. The system includes where first fuel hasa higher Reid vapor pressure than the second fuel. The system includeswhere the second fuel has a higher Reid vapor pressure than the firstfuel. The system includes where the first valve is a three-way valve,and further comprising a first fuel pump and a second fuel pump andadditional instructions to activate the first fuel pump and not activatethe second fuel pump in response to the indication of the impendingstart of the internal combustion engine.

In some examples, the system further comprises additional instructionsto estimate a Reid vapor pressure of the first fuel and a Reid vaporpressure of the second fuel. The system further comprises additionalinstructions to stop fuel flow through the second valve. The systemincludes where the fuel flow is stopped in response to an indicationthat the fuel rail is purged of the second fuel. The system furthercomprises additional instructions to pump the first fuel from the firstfuel tank to the second fuel tank in response to the second fuel beingin the second fuel tank a longer amount of time than the first fuel hasbeen in the first fuel tank, the second fuel being in the second fueltank for longer than a threshold amount of time.

Referring now to FIG. 3, a fuel system operating sequence according tothe method of FIGS. 4 and 5 is shown. The sequence of FIG. 3 may beprovided via the system of FIGS. 1 and 2 in cooperation with the methodof FIGS. 4 and 5. The plots of FIG. 3 are aligned in time. The verticallines at times t0-t8 represent times if interest in the sequence. The SSmarks along the horizontal axes represent a break in time and the breakin time may be long or short in duration.

The first plot from the top of FIG. 3 is a plot of engine operatingstate versus time. The vertical axis represents engine state and theengine state is off and not rotating when trace 302 is at a lower levelnear the horizontal axis. Trace 302 represents the engine operatingstate. The engine is on (e.g., combusting fuel) and rotating when trace302 is at a higher level near the vertical axis arrow. The horizontalaxis represents time and time increases from the left side of the figureto the right side of the figure.

The second plot from the top of FIG. 3 is a plot of Reid vapor pressure(RVP) of fuel in the first fuel tank (e.g., tank 202 of FIG. 2) versustime. The vertical axis represents the RVP value for fuel in the firstfuel tank and the RVP value increases in the direction of the verticalaxis arrow. Trace 304 represents the RVP value for fuel stored in thefirst fuel tank. The horizontal axis represents time and time increasesfrom the left side of the figure to the right side of the figure.

The third plot from the top of FIG. 3 is a plot of Reid vapor pressure(RVP) of fuel in the second fuel tank (e.g., tank 212 of FIG. 2) versustime. The vertical axis represents the RVP value for fuel in the secondfuel tank and the RVP value increases in the direction of the verticalaxis arrow. Trace 306 represents the RVP value for fuel stored in thesecond fuel tank. The horizontal axis represents time and time increasesfrom the left side of the figure to the right side of the figure.

The fourth plot from the top of FIG. 3 is a plot of engine temperatureversus time. The vertical axis represents engine temperature and enginetemperature increases in the direction of the vertical axis arrow. Trace308 represents the engine temperature. The horizontal axis representstime and time increases from the left side of the figure to the rightside of the figure.

The fifth plot from the top of FIG. 3 is a plot of an operating state ofa first fuel pump versus time. The vertical axis represents theoperating state of the first fuel pump and the first fuel pump isactivated and pumping fuel when trace 310 is at a level that is near thevertical axis arrow. The first fuel pump is not activated and notpumping fuel when trace 310 is at a level that is near the horizontalaxis. Trace 310 represents the first fuel pump operating state. Thehorizontal axis represents time and time increases from the left side ofthe figure to the right side of the figure.

The sixth plot from the top of FIG. 3 is a plot of an operating state ofa second fuel pump versus time. The vertical axis represents theoperating state of the second fuel pump and the second fuel pump isactivated and pumping fuel when trace 312 is at a level that is near thevertical axis arrow. The second fuel pump is not activated and notpumping fuel when trace 312 is at a level that is near the horizontalaxis. Trace 312 represents the second fuel pump operating state. Thehorizontal axis represents time and time increases from the left side ofthe figure to the right side of the figure.

The seventh plot from the top of FIG. 3 is a plot of an operating stateof a fuel supply valve (e.g., 206 of FIG. 2) versus time. The verticalaxis represents the operating state of the fuel supply valve and thefuel supply valve allows communication between the second fuel tank (T2)and the fuel rail (R) when trace 314 is near the level of T2-R along thevertical axis. The fuel supply valve prevents communication between thefirst fuel tank (T1) and the fuel rail (R) when trace 314 is near thelevel of T2-R along the vertical axis. The fuel supply valve allowscommunication between the first fuel tank (T1) and the fuel rail (R)when trace 314 is near the level of T1-R along the vertical axis. Thefuel supply valve prevents communication between the second fuel tank(T2) and the fuel rail (R) when trace 314 is near the level of T1-Ralong the vertical axis. Trace 314 represents the fuel supply valveoperating state. The horizontal axis represents time and time increasesfrom the left side of the figure to the right side of the figure.

The eighth plot from the top of FIG. 3 is a plot of an operating stateof a fuel return valve (e.g., 210 of FIG. 2) versus time. The verticalaxis represents the operating state of the fuel return valve and thefuel return valve allows communication between the fuel rail (R) and thesecond fuel tank (T2) when trace 316 is near the level of R-T2 along thevertical axis. The fuel supply valve prevents communication between thefuel rail (R) and the first fuel tank (T1) when trace 316 is near thelevel of R-T2 along the vertical axis. The fuel supply valve allowscommunication between the fuel rail (R) and first fuel tank (T1) whentrace 316 is near the level of R-T1 along the vertical axis. The fuelsupply valve prevents communication between the fuel rail (R) and thesecond fuel tank (T2) when trace 316 is near the level of R-T1 along thevertical axis. Trace 316 represents the fuel supply valve operatingstate. The fuel rail outlet valve is closed and prevents fluidiccommunication between the fuel rail (R) and the first fuel tank and thesecond fuel tank when trace 316 is at the level indicated as C. Thehorizontal axis represents time and time increases from the left side ofthe figure to the right side of the figure.

At time t0, the engine is stopped and the RVP of fuel in the first fueltank is at a high level. The RVP of fuel in the second tank is low andthe engine temperature is low. The first and second fuel pumps are notactivated. The fuel supply valve is positioned to allow fuel to flowfrom the second fuel tank to the fuel rail. The fuel return valve isclosed and it prevents flow of fuel to the first and second fuel tanks.Such conditions may be present when an engine has been off and has“soaked” at ambient temperatures.

At time t1, an indication is provided to give notice that an enginestart may be expected to occur shortly. The indication may be a vehicledoor that opens or a key fob, phone, or other remote device becomeswithin a threshold distance of the vehicle. Thus, before an engine startcommand is issued, an indication may be provided to generate additionaltime for purging a fuel rail of a lower RVP fuel. The engine remainsstopped and the RVP values for fuel in the first tank and fuel in thesecond fuel tank remain unchanged. The engine temperature remainsunchanged and the second fuel pump is not activated. The first fuel pumpin the first fuel tank is activated so that the fuel with the lower RVPmay be purged from the fuel rail before the engine is started. The fuelsupply valve also changes state to allow fuel to flow from the firstfuel tank to the fuel rail. The fuel return valve changes state to allowfuel to flow from the fuel rail to the second fuel tank. In this way,fuel with a lower RVP may be purged from a fuel rail before the engineis started. The RVP for fuels that are in the first and second fueltanks may be determined while the engine is stopped.

At time t2, the fuel return valve is closed so that fuel pressure maybuild in the fuel rail before the engine is started. The engine remainsoff and the RVP value for each of the fuels has not changed. The enginetemperature is unchanged and the first fuel pump remains activated. Thesecond fuel pump remains deactivated. The fuel supply valve ispositioned to allow fuel flow from the first fuel tank to the fuel rail.

At time t3, the engine is started with fuel from the first fuel tank.The RVP of the fuel in the first tank is unchanged and the RVP of thefuel in the second fuel tank is unchanged. The engine temperature beginsto increase and the first fuel pump remains activated. The second fuelpump remains deactivated and the fuel supply valve allows fuel to flowfrom the first fuel tank to the fuel rail. The sequence of FIG. 3 breaksin time at time t4.

The sequence resumes at time t5, where the engine is stopped. The RVP offuel in the first fuel tank is lower than the RVP of fuel in the secondfuel tank. The RVP value of fuel in a fuel tank may change due to addingfuel to the fuel tank or via evaporation of light ends or small chainhydrocarbons. The engine temperature is low and the first and secondfuel pumps are deactivated. The fuel supply valve is positioned topermit fuel to flow from the first fuel tank to the fuel rail. The fuelreturn valve is positioned to a closed position to prevent fuel fromflowing to the first fuel tank and the second fuel tank.

At time t6, an indication is provided to give notice that an enginestart may be expected to occur shortly. The indication may be a vehicledoor that opens or a key fob, phone, or other remote device becomeswithin a threshold distance of the vehicle. The engine remains stoppedand the RVP values for fuel in the first tank and fuel in the secondfuel tank remain unchanged. The engine temperature remains unchanged andthe first fuel pump is not activated. The second fuel pump in the secondfuel tank is activated so that the fuel with the lower RVP may be purgedfrom the fuel rail before the engine is started. The fuel supply valvealso changes state to allow fuel to flow from the second fuel tank tothe fuel rail. The fuel return valve changes state to allow fuel to flowfrom the fuel rail to the first fuel tank. In this way, fuel with alower RVP may be purged from a fuel rail before the engine is started.The RVP for fuels that are in the first and second fuel tanks may bedetermined while the engine is stopped.

At time t7, the fuel return valve is closed so that fuel pressure maybuild in the fuel rail before the engine is started. The engine remainsoff and the RVP value for each of the fuels has not changed. The enginetemperature is unchanged and the second fuel pump remains activated. Thefirst fuel pump remains deactivated. The fuel supply valve is positionedto allow fuel flow from the second fuel tank to the fuel rail.

At time t8, the engine is started with fuel from the second fuel tank.The RVP of the fuel in the first tank is unchanged and the RVP of thefuel in the second fuel tank is unchanged. The engine temperature beginsto increase and the second fuel pump remains activated. The second fuelpump remains deactivated and the fuel supply valve allows fuel to flowfrom the second fuel tank to the fuel rail.

In this way, fuel with a lower RVP may be purged from a fuel rail beforean engine start so that the engine may be started with a fuel that has ahigher RVP to improve cold engine starting. The fuel with the higher RVPmay be supplied to the engine and fuel rail whether the fuel with thehigher RVP is in the first fuel tank or in the second fuel tank. Thefuel with the lower RVP may be returned to the fuel tank that holds thefuel with the lower RVP.

Referring now to FIGS. 4 and 5, a method for operating a fuel system ofan internal combustion engine is described. The method of FIGS. 4 and 5may be incorporated into the system of FIGS. 1 and 2 as executableinstructions stored in non-transitory memory. The method of FIGS. 4 and5 may cause the controller of FIGS. 1 and 2 to receive inputs from oneor more sensors described herein and adjust positions or operatingstates of one or more actuators described herein in the physical world.

At 402, method 400 judges whether or not the engine is off (e.g., notrotating and not combusting fuel). If so, the answer is yes and method400 proceeds to 450. Otherwise, the answer is no and method 400 proceedsto 404. In one example, method 400 may judge that the engine is off iffuel is not being injected to the engine and engine speed is less than athreshold speed.

At 450, method 400 estimates the RVP of fuel in a first fuel tank andRVP of a second fuel in a second fuel tank. The fuel in the second fueltank may be of the same type (e.g., gasoline) as fuel in the first fueltank. In one example, method 400 estimates the RVP of fuels in the firstand second fuel tanks as described in U.S. Pat. No. 5,878,727, which ishereby incorporated by reference for all intents and purposes. Inanother example, method 400 estimates the RVP of fuels stored in thefirst and second fuel tanks as described in U.S. Pat. No. 9,850,853,which is hereby incorporated by reference for all intents and purposes.Method 400 stores the RVP estimates for fuels stored in the first andsecond fuel tanks to controller memory (e.g., RAM or ROM).

Additionally, method 400 may track an amount of time fuel has been inthe first and second fuel tanks, which may be referred to herein as thefuel's age. In one example, method 400 may record an amount of timebetween when fuel is added to each fuel tank to determine the amount oftime that fuel has been stored in the first and second fuel tanks. Forexample, if a fuel tank is nearly empty when the fuel tank is refilledat time t1, the amount of time that fuel has been in the fuel tankbegins at time t1 and it may end at a present time, for example at timet2. Thus, the age of the fuel in this example is the amount of timebetween time t2 and time t1. Further, an aggregate age of two mixedfuels may be determined as a function of the respective amounts of theindividual fuels and the age of the individual fuels. Method 400proceeds to 452 after estimating the age of fuel stored in the firstfuel tank and age of the fuel stored in the second fuel tank.

At 452, method 400 judges if a fuel has been stored in the first fueltank longer than threshold amount of time and if fuel has been stored inthe second fuel tank for less than a second threshold amount of time. Inone example, the threshold amount of time may be based on a type of fuel(e.g., gasoline, diesel, etc.) and other conditions of the fuel (e.g.,summer/winter blend). The second threshold amount of time may also bebased on the type of fuel and conditions of the fuel. If so, the answeris yes and method 400 proceeds to 454. Otherwise, the answer is no andmethod 400 proceeds to 460.

At 454, method 400 begins to add fuel from the second fuel tank to thefirst fuel tank by activating the second fuel pump in the second fueltank and adjusting the fuel supply valve (e.g., 206 of FIG. 2) such thatthe second fuel tank is in communication with the fuel rail and thefirst fuel tank is not in fluidic communication with the fuel rail viathe fuel supply valve. Method 400 also adjusts a position of the fuelreturn valve (e.g., 210 of FIG. 2) so that the fuel rail is in fluidiccommunication with the first fuel tank via the fuel return valve. Thus,the second fuel pump provides motive force to move fuel from the secondfuel tank to the first fuel tank so that the effective age of fuel inthe first fuel tank may be reduced, thereby improving combustionproperties of fuel stored in the first fuel tank. Method 400 maydeactivate the second fuel pump when a requested amount of fuel is movedfrom the second fuel tank to the first fuel tank. Method 400 proceeds to456.

At 456, method 400 may mix the two fuels that are now combined in thefirst fuel tank. In particular, method 400 may activate the fuel pump inthe first fuel tank and adjusting the fuel supply valve (e.g., 206 ofFIG. 2) such that the first fuel tank is in fluidic communication withthe fuel rail and the second fuel tank is not in fluidic communicationwith the fuel rail via the fuel supply valve. Method 400 may also adjusta position of the fuel return valve (e.g., 210 of FIG. 2) so that thefuel rail is in fluidic communication with the first fuel tank via thefuel return valve. Thus, fuel in the first fuel tank may be pumped up tothe fuel rail and returned to the first fuel tank via the fuel returnvalve to improve mixing of fuel that is in the first fuel tank. Method400 deactivates the first fuel pump after fuel in the first fuel tank ismixed for a predetermined amount of time. Method 400 proceeds to exit.

At 460, method 400 judges if a fuel has been stored in the second fueltank longer than threshold amount of time and if fuel has been stored inthe first fuel tank for less than a second threshold amount of time. Inone example, the threshold amount of time may be based on a type of fuel(e.g., gasoline, diesel, etc.) and other conditions of the fuel (e.g.,summer/winter blend). The second threshold amount of time may also bebased on the type of fuel and conditions of the fuel. If so, the answeris yes and method 400 proceeds to 462. Otherwise, the answer is no andmethod 400 proceeds to exit.

At 462, method 400 begins to add fuel from the first fuel tank to thesecond fuel tank by activating the first fuel pump in the first fueltank and adjusting the fuel supply valve (e.g., 206 of FIG. 2) such thatthe first fuel tank is in fluidic communication with the fuel rail andthe second fuel tank is not in fluidic communication with the fuel railvia the fuel supply valve. Method 400 also adjusts a position of thefuel return valve (e.g., 210 of FIG. 2) so that the fuel rail is influidic communication with the second fuel tank via the fuel returnvalve. Thus, the first fuel pump provides motive force to move fuel fromthe first fuel tank to the second fuel tank so that the effective age offuel in the second fuel tank may be reduced, thereby improvingcombustion properties of fuel stored in the second fuel tank. Method 400may deactivate the first fuel pump when a requested amount of fuel ismoved from the first fuel tank to the second fuel tank. Method 400proceeds to 464.

At 464, method 400 may mix the two fuels that are now combined in thesecond fuel tank. In particular, method 400 may activate the fuel pumpin the second fuel tank and adjust the fuel supply valve (e.g., 206 ofFIG. 2) such that the second fuel tank is in communication with the fuelrail and the first fuel tank is not coupled to the fuel rail via thefuel supply valve. Method 400 may also adjust a position of the fuelreturn valve (e.g., 210 of FIG. 2) so that the fuel rail is in fluidiccommunication with the second fuel tank via the fuel return valve. Thus,fuel in the second fuel tank may be pumped up to the fuel rail andreturned to the second fuel tank via the fuel return valve to improvemixing of fuel that is in the second fuel tank. Method 400 deactivatesthe second fuel pump after fuel in the second fuel tank is mixed for apredetermined amount of time. Method 400 proceeds to exit.

At 404, method 400 judges whether or not the engine is being coldstarted. If so, the answer is yes and method 400 proceeds to 406.Otherwise, the answer is no and method 400 proceeds to 430. In oneexample, method 400 may judge that the engine is being cold started ifengine temperature is less than a first threshold temperature and/or ifan exhaust catalyst temperature is less than a second thresholdtemperature.

At 430, method 400 judges if the RVP of fuel stored in the first fueltank is greater than the RVP of fuel stored in the second fuel tank.Method 400 may retrieve the RVP values for the respective fuels fromcontroller memory. The RVP values may be determined while the engine isoff as described at 450. If method 400 judges that the RVP of fuelstored in the first fuel tank is greater than the RVP of fuel stored inthe second fuel tank, the answer is yes and method 400 proceeds to 432.Otherwise, the answer is no and method 400 proceeds to 440.

At 432, method 400 activates the second fuel pump in the second fueltank. Method 400 may also adjust a position of a fuel supply valve suchthat the second fuel tank is in fluid communication with the fuel railvia the fuel supply valve. The first fuel tank is not in fluidcommunication with the fuel rail via the fuel supply valve. Method 400may also adjust a position of the fuel return valve such that the fuelrail is in fluidic communication with the second fuel tank for apredetermined amount of time before the engine is started. Method 400may close the fuel return valve after the predetermined amount of time.Method 400 proceeds to 434.

At 434, method 400 starts the engine via cranking the engine via astarter and injecting fuel from the second fuel tank into the engine.Method 400 proceeds to exit after the engine is started.

At 440, method 400 activates the first fuel pump in the first fuel tank.Method 400 may also adjust a position of a fuel supply valve such thatthe first fuel tank is in fluidic communication with the fuel rail viathe fuel supply valve. The second fuel tank is not in fluidiccommunication with the fuel rail via the fuel supply valve. Method 400may also adjust a position of the fuel return valve such that the fuelrail is in fluidic communication with the first fuel tank for apredetermined amount of time before the engine is started. Method 400may close the fuel return valve after the predetermined amount of time.Method 400 proceeds to 442.

At 442, method 400 starts the engine via cranking the engine via astarter and injecting fuel from the first fuel tank into the engine.Method 400 proceeds to exit after the engine is started.

At 406, method 400 judges if the RVP of fuel stored in the first fueltank is greater than the RVP of fuel stored in the second fuel tank.Method 400 may retrieve the RVP values for the respective fuels fromcontroller memory. The RVP values may be determined while the engine isoff as described at 450. If method 400 judges that the RVP of fuelstored in the first fuel tank is greater than the RVP of fuel stored inthe second fuel tank, the answer is yes and method 400 proceeds to 408.Otherwise, the answer is no and method 400 proceeds to 420.

At 408, method 400 activates the first fuel pump in the first fuel tank.Method 400 may also adjust a position of a fuel supply valve such thatthe first fuel tank is in fluidic communication with the fuel rail viathe fuel supply valve. The second fuel tank is not in fluidiccommunication with the fuel rail via the fuel supply valve. Method 400proceeds to 410.

At 410, method 400 may also adjust a position of the fuel return valvesuch that the fuel rail is in fluidic communication with the second fueltank for a predetermined amount of time before the engine is started.Method 400 may close the fuel return valve after the predeterminedamount of time. By allowing fluidic communication between the fuel railand the second fuel tank via the fuel return valve, fuel in the fuelrail with a lower RVP may be discharged to the fuel tank with the fuelhaving the lower RVP. Method 400 may adjust the position of the fuelline return valve based on the RVP of fuel that is in the fuel rail.Method 400 may determine the RVP of fuel in the fuel rail by storing tomemory the fuel that was being injected to the engine when the enginewas stopped. Thus, if fuel from the first fuel tank was being injectedto the engine when the engine was being stopped and fuel in the firstfuel tank has a higher RVP than fuel in the second fuel tank, thenmethod 400 may keep the fuel return valve closed. However, if fuel fromthe second fuel tank was being injected to the engine when the enginewas being stopped and fuel in the first fuel tank has a higher RVP thanfuel in the second fuel tank, then method 400 may adjust the position ofthe fuel return valve to direct fuel from the fuel rail to the secondfuel tank. Method 400 proceeds to 412.

At 412, method 400 starts the engine via cranking the engine via astarter and injecting fuel from the first fuel tank into the engine.Method 400 proceeds to exit after the engine is started.

At 420, method 400 activates the second fuel pump in the second fueltank. Method 400 may also adjust a position of a fuel supply valve suchthat the second fuel tank is in fluidic communication with the fuel railvia the fuel supply valve. The first fuel tank is not in fluidiccommunication with the fuel rail via the fuel supply valve. Method 400proceeds to 422.

At 422, method 400 may also adjust a position of the fuel return valvesuch that the fuel rail is in communication with the first fuel tank fora predetermined amount of time before the engine is started. Method 400may close the fuel return valve after the predetermined amount of time.By allowing fluidic communication between the fuel rail and the firstfuel tank via the fuel return valve, fuel in the fuel rail with a lowerRVP may be discharged to the fuel tank with the fuel having the lowerRVP. Method 400 may adjust the position of the fuel line return valvebased on the RVP of fuel that is in the fuel rail. Method 400 maydetermine the RVP of fuel in the fuel rail by storing to memory the fuelthat was being injected to the engine when the engine was stopped. Thus,if fuel from the second fuel tank was being injected to the engine whenthe engine was being stopped and fuel in the second fuel tank has ahigher RVP than fuel in the first fuel tank, then method 400 may keepthe fuel return valve closed. However, if fuel from the first fuel tankwas being injected to the engine when the engine was being stopped andfuel in the second fuel tank has a higher RVP than fuel in the firstfuel tank, then method 400 may adjust the position of the fuel returnvalve to direct fuel from the fuel rail to the first fuel tank. Method400 proceeds to 424.

At 424, method 400 starts the engine via cranking the engine via astarter and injecting fuel from the second fuel tank into the engine.Method 400 proceeds to exit after the engine is started.

In this way, operation of a fuel system may be adjusted to improveengine starting. In particular, during cold engine starts, a fuel with ahigher RVP may be supplied to the engine to improve engine starting.However, if the engine is started warm, a fuel with a lower RVP may besupplied to the engine so that fuel vaporization within the engine maybe within an expected range.

Thus, method 400 provides for operating a fuel system, comprising: inresponse to an indication of an impending engine start and an estimatedReid vapor pressure of a first fuel exceeding an estimated Reid vaporpressure of a second fuel, flushing the second fuel from a fuel rail toa second fuel tank with the first fuel supplied from a first fuel tankvia a controller. The method further comprises estimating the Reid vaporpressure of the first fuel and the second fuel when an engine thatincludes the fuel system is stopped. The method includes where theimpending engine start is a cold engine start. The method includes whereflushing the second fuel includes activating a first fuel pump thatsupplies the first fuel from the first fuel tank to the fuel rail. Themethod includes where the indication of the impending engine start is anopen vehicle door. The method includes where the indication of theimpending engine start is a fob being within a threshold distance of avehicle. The method further comprises pumping the first fuel from thefirst fuel tank to the second fuel tank in response to the second fuelbeing in the second fuel tank a longer amount of time than the firstfuel has been in the first fuel tank, the second fuel being in thesecond fuel tank for longer than a threshold amount of time.

Method 400 also provides for operating a fuel system, comprising: inresponse to a Reid vapor pressure, flushing a second fuel from a fuelrail to a second fuel tank with a first fuel supplied from a first fueltank via a controller; and pumping the first fuel from the first fueltank to the second fuel tank in response to a length of time the secondfuel has been in the second fuel tank. The method further comprisesmixing the first fuel with the second fuel via activating a second pumpin the second fuel tank and opening a fuel return valve. The methodincludes where the first fuel is pumped to the second fuel tank while anengine that the fuel system is coupled to is stopped. The methodincludes where pumping the first fuel tank to the second fuel tankincludes pumping the first fuel through a fuel rail. The method includeswhere the flushing occurs after an engine stop and before an enginestart.

Note that the example control and estimation routines included hereincan be used with various engine and/or vehicle system configurations.The control methods and routines disclosed herein may be stored asexecutable instructions in non-transitory memory and may be carried outby the control system including the controller in combination with thevarious sensors, actuators, and other engine hardware. The specificroutines described herein may represent one or more of any number ofprocessing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various actions,operations, and/or functions illustrated may be performed in thesequence illustrated, in parallel, or in some cases omitted. Likewise,the order of processing is not necessarily required to achieve thefeatures and advantages of the example examples described herein, but isprovided for ease of illustration and description. One or more of theillustrated actions, operations and/or functions may be repeatedlyperformed depending on the particular strategy being used. Further, thedescribed actions, operations and/or functions may graphically representcode to be programmed into non-transitory memory of the computerreadable storage medium in the engine control system, where thedescribed actions are carried out by executing the instructions in asystem including the various engine hardware components in combinationwith the electronic controller

This concludes the description. The reading of it by those skilled inthe art would bring to mind many alterations and modifications withoutdeparting from the spirit and the scope of the description. For example,I3, I4, I5, V6, V8, V10, and V12 engines operating in natural gas,gasoline, diesel, or alternative fuel configurations could use thepresent description to advantage.

The invention claimed is:
 1. A method for operating a fuel system,comprising: in response to an indication of an impending engine startand an estimated Reid vapor pressure of a first fuel exceeding anestimated Reid vapor pressure of a second fuel, flushing the second fuelfrom a fuel rail to a second fuel tank with the first fuel supplied froma first fuel tank via a controller.
 2. The method of claim 1, furthercomprising estimating the Reid vapor pressure of the first fuel and thesecond fuel when an engine that is coupled to the fuel system isstopped.
 3. The method of claim 1, where the impending engine start is acold engine start.
 4. The method of claim 1, where flushing the secondfuel includes activating a first fuel pump that supplies the first fuelfrom the first fuel tank to the fuel rail.
 5. The method of claim 1,where the indication of the impending engine start is an open vehicledoor.
 6. The method of claim 1, where the indication of the impendingengine start is a fob being within a threshold distance of a vehicle. 7.The method of claim 1, further comprising pumping the first fuel fromthe first fuel tank to the second fuel tank in response to the secondfuel being in the second fuel tank a longer amount of time than thefirst fuel has been in the first fuel tank, the second fuel being in thesecond fuel tank for longer than a threshold amount of time.
 8. Asystem, comprising: a first fuel tank; a second fuel tank; a first valvecoupled to the first fuel tank and the second fuel tank; a second valvecoupled to the first fuel tank and the second fuel tank; an internalcombustion engine including a fuel rail; and a controller includingexecutable instructions stored in non-transitory memory that cause thecontroller to route a first fuel from the first fuel tank through thefirst valve to the fuel rail and route a second fuel from the fuel railthrough the second valve to the second fuel tank in response to anindication of an impending start of the internal combustion engine, andadditional instructions to estimate a Reid vapor pressure of the firstfuel and a Reid vapor pressure of the second fuel.
 9. The system ofclaim 8, where the first fuel has a higher Reid vapor pressure than thesecond fuel.
 10. The system of claim 8, where the second fuel has ahigher Reid vapor pressure than the first fuel.
 11. The system of claim8, where the first valve is a three-way valve, and further comprising afirst fuel pump and a second fuel pump and additional instructions toactivate the first fuel pump and not activate the second fuel pump inresponse to the indication of the impending start of the internalcombustion engine.
 12. The system of claim 8, further comprisingadditional instructions to stop fuel flow through the second valve. 13.The system of claim 12, where the fuel flow is stopped in response to anindication that the fuel rail is purged of the second fuel.
 14. Thesystem of claim 12, further comprising additional instructions to pumpthe first fuel from the first fuel tank to the second fuel tank inresponse to the second fuel being in the second fuel tank a longeramount of time than the first fuel has been in the first fuel tank, thesecond fuel being in the second fuel tank for longer than a thresholdamount of time.
 15. A method for operating a fuel system, comprising: inresponse to a Reid vapor pressure, flushing a second fuel from a fuelrail to a second fuel tank with a first fuel supplied from a first fueltank via a controller; and pumping the first fuel from the first fueltank to the second fuel tank in response to a length of time the secondfuel has been in the second fuel tank.
 16. The method of claim 15,further comprising mixing the first fuel with the second fuel viaactivating a second pump in the second fuel tank and opening a fuelreturn valve.
 17. The method of claim 15, where the first fuel is pumpedto the second fuel tank while an engine that the fuel system is coupledto is stopped.
 18. The method of claim 15, where pumping the first fuelfrom the first fuel tank to the second fuel tank includes pumping thefirst fuel through a fuel rail.
 19. The method of claim 15, where theflushing occurs after an engine stop and before an engine start.